(1) Field of the Invention
This invention relates to color display devices, in particular to pseudomorphic cladded quantum dot nanophosphor thin film color display devices.
(2) Description of the Related Art
Cladded quantum dots have been extensively reported in the literature as a way of improving light emission intensity in quantum dots. For example, cladded nanocrystals (CNCs) having CdSe core and ZnS cladding (or shell) have been used to produce high photoluminescence efficiency quantum dots. These dots however, suffer from appreciable lattice mismatch between the core and cladding layer(s) that in turn contributes to a number of undesirable features such as:    i) Introduction of interfacial states responsible for photoluminescence (PL) and electroluminescence (EL) quenching.    ii) Limitation in growing thick cladding layers, as a result of lattice mismatch which becomes more pronounced in luminescence quenching as the cladding thickness increases.    iii) The blinking behavior of these CNCs, closely related to exciton confinement in these nanoparticles, is greatly affected by defects such as that introduced by the lattice mismatch between core and cladding layer(s).    iv) Quantum dot devices require incorporation of dots in thin film forms that can be used for electronic excitation. The size distribution, composition variation and non-periodic placement of these quantum dots greatly affect the emitted wavelength, and overall device performance, especially for lasing. Growth of quantum dots by self-organization has been reported to demonstrate lasing. However, these devices do not yield the expected improvement in the threshold current density predicted for quantum dot lasers.
Liquid phase (solution) grown nanocrystals possess certain limitations in creating core-shell architectures with improved lattice-matching. Although the lattice mismatch of ZnS cladding is significant to that of the CdSe core, these CNCs emit brighter than their uncladded counterparts. Cladding with CdS produces less strained lattices. This in turns makes the CdSe/CdS core/shell CNCs to photoluminescence brighter than the CdSe/ZnS CNCs. Restricting the cladding layer thickness to only few monolayers provides one way to reduce the interfacial defects (via strained layers). Once the thickness of the cladding is increased, the photoluminescence quantum efficiency of these CNCs plummets as a result of strain-induced defects, which become dominant. Therefore, the present state of the art is based on thinly cladded CNC nanocrystals. These CNCs are further coated with surface passivation organic surfactant layers, which in turn play a major role to the photoluminescence efficiency of these nanocrystals. Nanocrystals prepared this way, when incorporated in electroluminescent (EL) devices, utilizing carrier injection invariably yields poor EL efficiencies.
A layer of trioctyl phosphine oxide (TOPO) is presently one of the best surface passivating agent used in the liquid phase growth of these CNCs. TOPO in conjunction with TOP (trioctyl phosphine) passivates both cationic (Cd2+, Zn2+, Te2+, etc.) and anionic (Se2−, S2−, O2−, etc.) surface species respectively. FIG. 1(a) shows a typical liquid phase grown cladded nanocrystal with a core (1) and a thin cladding (2). The outer cladding layer 3 comprises of both TOPO and TOP. FIG. 1(b) shows a single pixel EL structure utilizing these nanocrystals in the form of a thin layer (7) sandwiched between a hold transporting organic layer (6) and top metal electrode layer (8). The hole-transporting layer 6 is realized on a thin transparent ITO (indium tin oxide) electrode (layer 5), supported on a substrate (4). Poly (p-phenylenevinylene) along with other hole injecting organics has been used with limited success.
Once the organic passivation agent(s) are partially removed or somehow degrade during device operation, the emission characteristics and brightness of these CNCs are significantly impaired. In addition, the natural inability of the organic phase towards dielectric breakdown poses a significant limitation for these CNCs to find application in field-assisted electroluminescent devices.
An alternate technology is the use of doped nanocrystals, DNCs, where a dopant is introduced in the quantum dot, such as Mn in ZnS. These DNCs are incorporated in the form of thin films and used for electroluminescence or cathodoluminescence device applications.
Use of cladding layer material that has a higher band gap than the core with similar lattice constant results in a pseudomorphic cladded quantum dot. In this arrangement, the core-cladding lattice mismatch is accommodated as tensile or compressive strain. Having a small mismatch permits the growth of thicker cladding layers as has been shown in pseudomorphic strain layer quantum-wells. This improves the confinement of holes, electrons and excitons inside the core. In addition, these CNCs are less affected by the environment of the outer surface of the cladding layer, thus putting less stringent requirements on additional surface modifying layers. In particular, the known susceptibility of chalcogenides and other semiconducting materials to form oxides in the presence of atmospheric moisture and oxygen justifies the growth of compatible surface passivation layer(s) to further improve their environmental stability. Alternatively, one can use a compatible higher energy gap semiconducting layer before applying the outer passivation layer. Depending on the device structure, the outer layers could be semiconducting or conducting.
Quantum dots have been used to realize nanophosphor for a variety of display, lasers and luminescent-based diagnostic applications. The luminescent properties of these dots however, are strongly influenced by surface states. Cladding layers have been used to reduce these surface states, although the lattice mismatch in the materials for core and cladding creates undesirable quenching defects. In addition, the cladding layer prevents the injection of carriers, with the holes especially, hindered in reaching the core of these nanophosphors.